Metastasis causes >90% of breast cancer-related deaths. Tumor cell structures that have been hypothesized as necessary for metastasis are invadopodia, protrusions rich in structural proteins (e.g. Tks5), adhesion proteins (e.g. integrin ?1) and proteases (e.g. MT1-MMP), known to degrade the extracellular matrix (ECM) proteins. A novel, real-time imaging methodology that allows investigation of the in vivo metastatic role of invadopodia found that cells which assemble invadopodia move at ?Slow? speeds and are seen in perivascular niches. Outside of perivascular niches, ?Fast? motile cells, which do not assemble invadopodia were observed. This method also showed that invadopodia are essential for intravasation in vivo. Preliminary work has begun to determine which aspects of the Slow phenotype allow invadopodia assembly. Use of novel mathematical models, in vitro microscopy and novel computational image analysis demonstrates that whereas the Fast cells continuously locomote, the Slow cells are in either of two oscillating states: i. Invadopodia state, in which a cell is sessile while it degrades surrounding ECM; ii. Locomotion state, similar to a Fast cell. The oscillation dynamics appear to depend on interactions between the ECM and its receptor integrin ?1. Importantly, the Invadopodia, but not Locomotion state, is limited to G1 phase of the cell cycle. Release from G1-arrest amplifies invadopodia, implying that G1-arresting therapies may promote metastasis. This project will identify the mechanisms unique to the Invadopodia state compared to cells that only proliferate or locomote, with the idea of targeting the Invadopodia state. Overall Hypothesis: the Invadopodia state, which is essential for metastasis, requires a pause in both cell locomotion and cell cycle progression. The initiation and termination of the Invadopodia state are controlled by the interaction between the ECM, invadopodia components integrin ?1 and Tks5, as well as the cyclin-dependent kinase inhibitor p27 (expressed during G1). Aim 1. Determine the role of the cell cycle in regulating the Invadopodia state. Using real-time imaging of cell cycle and invadopodia markers in vitro and in vivo, this Aim tests the hypothesis that cyclin- dependent kinases and corresponding inhibitors regulate the Invadopodia state, but not the Locomotion. Aim 2. Determine the role of ECM-cancer cell interactions in regulating the oscillations between Invadopodia and Locomotion states. This Aim will provide a strategy based on in vivo modification of integrin ?1 activity on how to turn Invadopodia state ?off? towards metastatic prevention. Significance. Invadopodia is suggested as a new candidate target for metastatic prevention. The use of cell cycle inhibitors in patients with invadopodia may be accelerating metastatic dissemination. The long- term goal is to predict and prevent metastasis using invadopodia.